The present invention relates to holding devices, and, more particularly, to an apparatus for holding a rotary tool.
Rotary working devices have long been available that utilize removable rotary bits or other working tools as sanding heads, grinding heads, or the like. A rotary working device includes a collet nut that grips the working tool during operation of the working device. In a known collet nut a collet is carried in a recess between a collet nut and an end portion of a spindle driven by a motor. Typically, the collet nut includes internal threads that engage external threads of the spindle. Rotation of the collet nut causes the nut to advance toward or retract from the spindle, thereby changing the axial size of the recess in which the collet is located and thereby causing the spacing between collet teeth to change so that working tools of various diameters can be accommodated.
Yorde U.S. Pat. No. 6,155,576, discloses a collet disposed in a recess between a spindle and a collet nut threaded onto the spindle. The collet includes four segmented jaw portions each carried by a shank portion. The shank portions are, in turn, integral with a base portion. The base portion is disposed at a first end of the collet and the jaw portions are disposed at a second end of the collet. Each of the jaw portions includes a first frusto-conical camming surface at the first and of the collet and a second frusto-conical camming surface axially spaced from the first frusto-conical camming surface and disposed at a central portion of the collet intermediate the first and second ends thereof. When the collet nut is threaded on the spindle, the first and second frusto-conical camming surfaces of the jaw portions ride on an inner frusto-conical camming surface of the collet nut and a chamfered end surface of the spindle, respectively, and the jaw portions are moved radially inwardly or outwardly.
One disadvantage encountered in the Yorde device results from the process of forming the collet. Specifically, outer surfaces of the collet are machined from round bar stock and a central bore of the collet is drilled through the jaw segments. In the disclosed example, a uniform hole diameter of 0.080 inches is formed for a xe2x85x9th inch maximum tool shank diameter. Thereafter, first and second longitudinal slots are sequentially cut to define the jaw portions. The jaw portions are then spread and the collet may or may not be heat treated. In the disclosed example, this spreading causes the diameter of the bore at the first end of the collet to increase to about 0.130 inch and the diameter at approximately the central portion of the collet to increase to about 0.110 inch. According to the Yorde patent, the collet is said to be capable of clamping and holding a tool shank in a range between {fraction (1/32)}nd inch and xe2x85x9th inch diameter. However, in practice, this device has been found to encounter holding problems during use. These problems arise, in part, from the lack of precision in cutting the slots (particularly the second slot due to the lack of rigidity caused by the existence of the first slot) and are further due to mismatches between the radii of curvature of the tool shank and the central bore of the collet and mismatches between the frusto-conical camming surfaces and the chamfered end surface of the spindle.
FIGS. 1 and 2 illustrate simplified cross sections of the Yorde device under a first condition wherein a tool shank 4 of 0.125 inch diameter is inserted in a central bore 5 originally drilled to a 0.094 inch diameter and wherein each of first and second slots 6, 7 are off center by 0.002 inch. As can be seen by a careful examination of FIGS. 1 and 2, the difference in radii of curvature of the collet nut 9, as represented by the curved line 12, and the outer surfaces of the jaw portions 14, as represented by a curved line 16, results in contact of the collet nut 9 with each of the jaw portions 14 at a central section 17. In addition, each jaw portion 14 contacts the tool shank 14 at a highly localized area 18 (the area 18 is essentially a line of contact extending along the length of the tool shank 14). This condition results in an off-center application of highly localized holding forces on the tool shank 14. A similar condition arises during contact of the chamfered end surface of the spindle with the second frusto-conical camming surfaces of the jaw portions. Typically, these conditions result in run out and frequent release of the tool.
An opposite but analogous condition typically arises when the tool shank diameter is smaller than the diameter of the hole drilled to form the central bore. Specifically, the collet nut 9 contacts each of the jaw portions 14 at one or more locations located circumferentially outside of the area of contact of each of the jaw portions with the tool shank 14. Again, an off-center holding force condition arises, resulting in run out and release of the tool.
A further difficulty with the Yorde device results from the fact that the inner frusto-conical camming surface of the collet nut and the chamfered end surface of the spindle (and the mating surfaces of the jaw portions) are disposed at greatly different angles with respect to a longitudinal centerline of the tool. This results in differential bending moments being applied to the jaw portions 14 and prevents the development of uniform holding forces on the tool along the length thereof. A further difficulty with the Yorde device results from the difference in hardness between the hardened collet and the collet nut and spindle. Specifically, the collet nut and spindle are commonly made from 12L14 steel, which is a soft material at a Rockwell hardness of B90. The difference in hardness between the collet and the collet nut and spindle may result in the collet tearing material away from the tapered portions of the collet nut and spindle, causing permanent damage to the components and adversely affecting the assembly accuracies.
In accordance with one aspect of the present invention, a tool holding apparatus includes a spindle and a collet nut engaged with a spindle wherein a recess is defined between the spindle and the collet nut. A collet is disposed in the recess and includes a first tapered portion and a second tapered portion spaced from the first tapered portion, wherein the first and second tapered portions form substantially equal angles with respect to a longitudinal center line of the collet.
According to a further aspect of the present invention, a combination includes a spindle, a collet nut engaged with the spindle wherein a recess is defined between the spindle and the collet nut and a collet disposed in the recess. A tool is engaged by the collet and the collet includes a plurality of fingers each having first and second tapered portions. The collet nut includes a third tapered portion that engages the first tapered portion of each finger at first and second contact areas associated with such finger. In addition, each finger engages a shank of the tool at a third contact area associated with such finger. A first line extending between the first and third contact areas associated with a finger forms an angle less than or about equal to 32 degrees with respect to a second line extending between the second and third contact areas associated with the same finger.
In accordance with yet another aspect of the present invention, a method of holding a tool having a tool shank wherein the shank has a diameter less than or substantially equal to a particular diameter comprises the steps of providing a section of stock and drilling a bore of the particular diameter through the section of stock. First and second slots are formed in the section of stock from a first end thereof extending toward but not fully to a second end thereof wherein the slots define a plurality of fingers, each finger including a main portion and a shank portion connecting the main portion to a base portion. The section of stock is placed in a recess between a collet nut and a spindle and the tool shank is inserted into the drilled bore of the section of stock. The collet nut is engaged with the plurality of fingers so that the fingers bear against and retain the tool shank therebetween.
Other aspects and advantages of the present invention will become apparent upon consideration of the following detailed description.